Substrate-based chip packages are also referred to for example as BGA packages, BGA standing for Ball Grid Array. U.S. Pat. No. 6,048,755, which is incorporated herein by reference, teaches a method for producing a BGA package using a substrate with a patterned solder resist mask.
In the case of such substrate-based chip packages, it is very important that care is taken to protect all the edges of the chip, since cracks or other mechanical damage can also have effects on the active chip side. Such damage may arise in the course of handling, during the back-end process or else with the customer. In order to avoid this, a molded cap (covering material or molding compound), which envelops the rear side of the chip and adjacent regions of the substrate, may be used.
For durable functioning of the chip package, it is of special significance that the covering materials have very good adhesive properties with respect to the surfaces of the different materials of the package. A particular weak point in the package in this case is the solder resist, which for reasons of minimizing warpage is applied on both sides. That is to say, it is also applied on the chip side. Warpage is to be understood as meaning that in a layered structure a distortion of the substrate occurs under the influence of temperature unless layers of different coefficients of expansion are present on both sides, or at least are irregularly distributed.
The solder resist not only absorbs a relatively large amount of moisture (up to about 8%), which is unfavorable for the reliability characteristics of the package, but also significantly reduces the adhesion of the die-attach material and of the molding compound on the substrate base material. In addition, it is also very difficult to harmonize the adhesive properties of the molding-compound and die-attach materials (adhesive) over the entire temperature range (−65° C. to 150° C. in the extreme case). A so-called CTE (coefficient of thermal expansion) mismatch arises, as a result of which the reliability characteristics are significantly reduced.
Furthermore, the moisture content of the solder resist can lead to problems of reliability as a consequence of the higher soldering temperatures due to the use of lead-free solder materials.
The consequence is that the packages may already delaminate, that is come apart, at the molding compound/solder resist, solder resist/substrate and solder resist/die-attach material interfaces during preconditioning.
In order to achieve an improvement in this, the chip is either molded around completely (TSOP), provided with molded caps (for example, BOC with backside protection or BSP) or with other mechanical coverings on the module level. In the case of U.S. Pat. No. 6,048,755, it was attempted to solve the problem at least partly by removing the solder resist under the chip.
In the case of the molded cap, it was attempted to solve the problem by optimizing the adhesive force on the solder resist, which however did not lead to optimum reliability characteristics.